In recent years, electronic devices using a liquid crystal display have been reduced in weight. For example, the liquid crystal display is used in information devices such as mobile phones and mobile PCs. Regarding the operation of information devices, for example, a technology to enable direct input to a liquid crystal display screen by a finger, a pointer or the like is applied.
The methods of direct input to the liquid crystal display screen include an on-cell type, in which a touch panel having a sensing function is arranged in front of a liquid crystal panel and input is accepted by the touch panel, and an in-cell type, in which the sensing function is formed in an array substrate or a counter substrate of a liquid crystal display as a sensor arranged in a matrix shape and installed inside a liquid crystal cell.
Patent Literature 1 (Jpn. Pat. Appln. KOKAI Publication No. 10-171599) discloses touch panels of the resistance film type, electromagnetic induction type, capacitance type, and optical type as technologies used for the on-cell type. However, according to the on-cell type in which a touch panel is disposed on the surface of a liquid crystal panel, the thickness and weight of the touch panel are added to those of a liquid crystal display and so causes an increase in thickness and weight. Further, due to light reflex by the surface of the touch panel and an inner surface of the touch panel, a liquid crystal display quality may deteriorate.
In contrast, the in-cell type in which a sensor is installed inside a liquid crystal cell as a liquid crystal display incurs no increase in thickness of the liquid crystal display and is unlikely to cause degradation of display quality, which makes this type preferable. The development of an optical sensor as a sensor having the sensing function is in progress.
In a liquid crystal display used for information devices, stereoscopic image display is increasingly being used and technical demands, for example, a demand to provide a sense of clicking on the display of a button to which a stereoscopic display effect is attached, error prevention during finger input and the like are increasing. For the finger input, a type in which a touch panel is externally attached to the surface of a liquid crystal display is known. An input type using an optical sensor contained in a liquid crystal panel to achieve weight reduction is under development. A liquid crystal display containing the optical sensor is subjected to the temperature and a backlight light source and may need compensation for the optical sensor.
In a silicon photodiode using polysilicon or amorphous silicon as a channel layer, a dark current may be generated by changes in the environmental temperature or the like, which adds noise that is not observed light to measured data. In a silicon photodiode having the grain boundary of a liquid crystal such as polysilicon, continuous-grain silicon or the like, positional variations of the grain boundary directly lead to variations of photodiode characteristics, which makes it difficult in some cases to form a plurality of uniform optical sensors in the screen of a liquid crystal display. When compared with this photodiode, phototransistor characteristics of an optical sensor from an oxide semiconductor described later are extremely uniform.
Patent Literature 2 (Jpn. Pat. Appln. KOKAI Publication No. 2002-335454) and Patent Literature 3 (Jpn. Pat. Appln. KOKAI Publication No. 2007-18458) disclose a technology to make operation corrections by using a photodiode that corrects a dark current. Patent Literature 2, 3 disclose a dark current correction technology by an image sensor, but do not disclose stable input when a phototransistor formed from an oxide semiconductor is applied to a display apparatus and a technology to control noise originating from reflected light. A photo-sensor formed from an oxide semiconductor does not have a large dark current accompanying a photo-sensor formed from a silicon semiconductor and there is no need to actively perform dark current correction.
Patent Literature 4 (WO 2009/116205) discloses a technology to emit a sensing dedicated light from an oblique direction to achieve stable input when a photoreceptor is used as a touch sensor. However, Patent Literature 4 does not disclose a technology to control noise originating from reflected light in a liquid crystal cell, that a photoreceptor of an oxide semiconductor having uniform characteristics with less characteristic variations among a plurality of elements is used, or realization of a more stable input technology by use of a signal-compensated photoreceptor. According to the technology of Patent Literature 4, the sensing dedicated light is always emitted in a different direction from a direction of the observer via a slit of a light shielding layer and the dedicated sensing light may enter an eye of the observer due to a chipped portion of a black matrix, irregular reflection from TFT (thin film transistor) metal wires, or diffraction of light, leading to display degradation. Further, Patent Literature 4 does not disclose that an intensity of an oblique emission light is switched depending on the purpose (purpose of image quality priority, security, or finger input) of using a liquid crystal display and the reduction of signal variations by reflected light increased due to a shade difference (luminance difference) of the image display.
In recent years, an oxide semiconductor called IGZO has received widespread attention. An oxide semiconductor whose band gap is high, i.e., 2.5 to 3.5 ev when compared with a silicon photodiode, has an extremely small dark current and so the necessity of compensation to reduce a dark current as in the above silicon photodiode is low. Further, a phototransistor in which a transparent channel layer is formed from an oxide semiconductor has uniform characteristics with less variations even if a plurality of such phototransistors with a large area is formed. Based on such a viewpoint, technological development of using an oxide semiconductor as an optical sensor is under way.
Patent Literature 5 (Jpn. Pat. Appln. KOKAI Publication No. 2010-186997) and Patent Literature 6 (Jpn. Pat. Appln. KOKAI Publication No. 2011-118888) disclose an optical sensor (photoreceptor) technology using an oxide semiconductor.
Patent Literature 5 discloses an optical sensor technology applied to a display mainly using organic matter for a luminescent layer. Patent Literature 6 relates to a display apparatus including an optical sensor for position detection, in addition to an optical sensor as an area sensor. Neither Patent Literature 5 nor Patent Literature 6 discloses a liquid crystal drive technology to emit oblique light.